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1. Field of Invention
This invention relates generally to contact lenses, particularly to a bifocal contact lens in which at least one primary prism and one secondary prism are present.
2. Prior Art
Bifocal contact lenses are lenses comprised of two or more areas with different optical powers, often referred to as zones. A far-power zone provides the optical power for a wearer""s distance vision and a near-power zone, sometimes called a segment, provides the optical power for a wearer""s near vision. The two zones may be subdivided into additional zones of different powers or combined into a variable power zone, in which case a bifocal contact lens may also be called a multifocal lens as in U.S. Pat. Nos. 5,517,260 and U.S. Pat. No. 5,754,270, but the same principles described as follows will apply.
The retinal image and the visual percept that results from it are dependent upon the light that enters the eye through the entrance pupil. In order for a bifocal contact lens to function properly the entrance pupil must be covered at least partly or, more effectively, completely by the far-power zone of the lens when the eye observes a distant object and covered at least partly or, more effectively, completely by the near-power zone of the lens when the eye observes a near object. This function can be accomplished by the principle of alternating vision in which a vertical shifting action or translation of the contact lens is made to occur in order to place one or the other zone in front of the entrance pupil as the eye alternates between viewing distance and near objects (Mandell, 1988; Ruben and Guillon, 1994).
Alternatively, a principle known as simultaneous vision can be utilized whereby the contact lens is designed and fitted in such a way as to position part or all of both the far and near-power zones in front of the entrance pupil at the same time so that each contributes to the retinal image simultaneously. In the latter case the lens requires little or no vertical shifting action. Simultaneous vision has the drawback, however, that two images are seen at once and at least one of the two retinal images formed by light from the distance and near zones is always out of focus so that vision is compromised (Mandell, 1988; Ruben and Guillon, 1994).
Bifocal contact lenses generally are classified into two types, segmented and concentric. Segmented bifocal contact lenses have two or more vertically separated power zones that function usually but not always by the alternating vision principle as described in U.S. Pat. Nos. 3,597,055 and 3,684,357. Concentric bifocal contact lenses have a central power zone and one or more annular power zones that function usually, but not always, by the simultaneous vision principle., as described in U.S. Pat. Nos. 4,636,049: 4,752,123, 4,869,587 and 5,864,379. The far and near-power zones, together with optional transition curves, comprise the bifocal area. The remainder of the lens surface is comprised of one or more options that are used to complete the lens shape, such as lenticular curves, truncations, slab off, and various edge contours.
There are several subtypes of segmented bifocal contact lenses, based on the shape of the near-power zone, including round, D-shaped, flat, crescent, and others as described by Conklin Jr. et al, 1992 and in U.S. Pat. No. 5,074,082. The near-power zone is usually placed at the lower portion of the lens and maintained in that position by various features that can be incorporated into the lens in an attempt to control the lens position and stabilize the meridional rotation as described in U.S. Pat. Nos. 4,095,878; 4,268,133; 5,760,870; 5,296,880; and 4,573,775. This is commonly accomplished in rigid bifocal contact lenses by incorporating a prism into the lens, in which case the lower part of the lens is relatively thick and the upper part relatively thin. The prism serves to maintain the desired lens orientation and keep the near-power zone of the lens downward, as described in U.S. Pat. Nos. 5,430,504 and 4,854,089 and in Burris, 1993; Bierly, 1995, and Conklin Jr. et al, 1992.
In rigid prism bifocal contact lenses the lower edge of the lens tends to rest on the upper margin of the lower lid. When the wearer views a distant object the far-power zone ideally is positioned to cover the entrance pupil of the eye, and the near-power zone is positioned below the entrance pupil by gravity and the downward force of the upper lid (Forst, 1987). For reading or other near-vision tasks, the eye rotates downward and the contact lens ideally shifts upward relative to the eye, moving the near-power zone to a position in front of at least part of the entrance pupil in order to provide an optical correction for near vision.
Attempts have been made to design a soft prism bifocal contact lens that has a construction similar to that used in rigid lenses. However, soft bifocal contact lenses are much larger that rigid contact lenses, usually between 13 and 15 mm, and often extend beyond the limbus of the eye. When a soft prism bifocal contact lens is worn, the thicker portion of the lens moves downward and lies beneath the lower lid. As a result, the lens is not supported or braced by the upper margin of the lower lid. Hence, the prism component is successful in moving a soft prism bifocal contact lens to the desired low position and controlling meridional rotation but is not successful in inducing a vertical lens shift as the eye looks back and forth between distant and near objects.
Despite some early reports of successes, present soft prism bifocal contact lenses do not have sufficient vertical shifting action to fulfill the alternating vision principle and thus do not provide acceptable vision for both distance and near viewing (Mandell, 1988; Ruben and Guillon, 1994). Several prism bifocal soft contact lenses have been developed and brought to the marketplace, only to be later discontinued. Soft contact lenses of this type have been described in U.S. Pat. Nos. 4,549,794; 5,635,998; 5,635,998; 4,618,229; and 5,141,301. To the best of my knowledge, there is no soft bifocal contact lens that functions by the alternating vision principle and provides optimal vision.
Most soft bifocal contact lenses that are available today are of the concentric bifocal type and operate on the principle of simultaneous vision (Norman, 1995; Burris, 1993). It is recognized that these lenses do not provide good vision for both distance and near viewing and are only worn successfully by those who are willing to accept less than optimal vision.
Attempts have been made to induce a vertical shift of a soft bifocal contact lens by adding features to the lower periphery of the lens. U.S. Pat. No. 6,109,749 describes a soft bifocal contact lens that has an integrally formed bevel to aid translation of the lens. The bevel portion has upper and lower shoulders which converge to form an extended bevel. The bevel does not form part of the optical portion of the lens. This lens may have a single prism incorporated into the structure. U.S. Pat. No. 5,635,998 shows a multifocal contact lens that has an ellipsoidal shape and a single prism, which in combination produce an elongated zone of contact between the base portion of the prism and the lower eyelid. U.S. Pat. No. 5,912,719 and European Pat. No. 0 042 023 show lenses that are comprised of palpebral (lid) bosses projecting locally from the external surface in the peripheral area, where each palpebral boss is globally elongate in a circumferential direction and with a crest line of limited dimensions. The crest line has a peak in its middle area. U.S. Pat. No. 4,614,413 describes a lens with a lenticular carrier which serves as a palpebral anchor.
Previous contact lenses have been made that contained more than one prism but for the purpose of accomplishing a smooth transition between the distance and near zones in the bifocal area. U.S. Pat. No. 4,854,089 describes a bifocal contact lens in which a first prism component is incorporated into the base curve and a second prism component is incorporated into the distance vision curve. The second prism is not designed so as to enhance lens shifting or translation and does not extend forward at the base of the second prism in order to contact the lower lid and increase the function of lens shifting.
The vertical shifting deficiency in previous soft prism bifocal contact lenses is corrected in my invention by incorporating two or more prisms into the same lens, which operate together but with different structure and function. One or more primary prisms provide a desired lens vertical positioning on the eye during distance viewing and control meridional rotation in the plane of the corneal perimeter, the limbus. In addition, one or more secondary prisms with bases that extends forward from the adjacent lens surface provide vertical lens shifting, a translation movement, so that the desired optical power zone of the contact lens is moved in front of the entrance pupil of the eye at the desired time. The lens usually contains a segmented bifocal area on one surface in which the far-power zone of the lens is uppermost and the near-power zone lowermost in position but a concentric bifocal may be used as can any other bifocal construction for which a shifting or translational movement of the contact lens on the eye will change the power effect within the bifocal area.
It is an object of my invention to provide a soft bifocal contact lens that will give optimal vision to the wearer for viewing both distance and near objects as the result of improved lens shifting.
It is a further object of my invention to make a bifocal contact lens of soft or flexible materials so as to provide maximum comfort to the wearer.
It is a further object of my invention to provide a soft bifocal contact lens that will take advantage of the anatomical structure of the eye and lids for purposes of initiating a vertical shifting or translational movement, so as to provide one or another optical power to the eye at the desired time.
It is a further object of my invention to provide a bifocal contact lens that will have better image forming properties than that of previous soft bifocal contact lenses.
It is a further object of my invention to provide a soft bifocal contact lens that is relatively inexpensive to manufacture using standard lathing or molding techniques.
It is a further object of my invention to expand the number of patients who are able to wear bifocal contact lenses
The advantage of my soft bifocal contact lens is to provide the wearer optimal vision for both distance and near vision with good comfort.
Further objects and advantages of my invention will become apparent from a consideration of the drawings and ensuing description.